When gratings are formed in optical fibers to a depth wherein evanescent waves are encountered, the grated optical fibers can be used as sensors, filters and the like. For example, the optical fiber can act as a reflector such as a Bragg reflector. Pairs of these gratings can function as interferometers. When several pairs of gratings which reflect light at different wavelengths are coupled together on a single fiber, the fiber can be used as a seismic sensor to make geophysical measurements.
Since the reflectors can be implemented on a fiber, very compact sensors can be manufactured. Therefore, numerous sensors can be employed to enhance the repetitiveness and accuracy of the seismic measurements. The gratings can be formed in accordance with the teachings of Optics Letters, Vol. 3, No. 2, pp. 66-68, August 1978, incorporated herein by reference.
A preferred method of forming the gratings is disclosed in "A Process of Fabricating a Portion of an Optical Fiber Capable of Reflecting Predetermined Wavelength Bands of Light" by Donald C. Schmadel, Jr., filed concurrently herewith. The teachings of that application are incorporated herein by reference and assigned U.S. application Ser. No. 546,608, filed in Oct. 28, 1983. This processing technique enables the manufacture of gratings of varying reflectivity. The gratings can be used as reflectors to filter undesirable wavelengths of light from the passage through the fiber. Pairs of grated optical fiber sections having the same reflective characteristics can be used as interferometers. Pairs of gratings are defined as sections of the fiber which contain gratings and are separated by a length of non-grated optical fiber.
However, either process of forming the gratlngs tends to weaken the fiber or make it more susceptible to stretching. These handling problems are compounded because as many as sixty or more separate gratings, i.e., thirty or more pairs, are needed on a fiber when it is used as a hydrophone for seismic exploration. Since the distance between the gratings is critical to maintain a specific reflectivity, a stretched fiber is wholly unsuitable for a specific filtering task. In addition, in actual applications, the gratings are subject to damage by breaking as well as harmonic movement from air currents which can shift the frequency of the light traveling through the fiber. Thus, it would be highly desirable to have a ruggedized grated optical fiber which is not effected by ambient conditions. In addition, it would also be desirable to have a fiber which is protected from stretching which can destroy the specific wavelengths at which light is reflected.